INHIBITION OF SUCCINATE OXIDATION 57 



bition of succinate dehydrogenase when the enzyme is isolated from the 

 cells. It will be well to mention some of these in order to emphasize that 

 there are usually many ways, other than by permeability, by which unex- 

 pected phenomena may be explained. 



(a) The enzyme environment within the cell is different from the artificial 

 media used with isolated enzymes (see Chapter 1-9). Many substances may 

 be able to alter malonate inhibition; we have noted the effects of Ca+"'" 

 and ATP. The concentrations of such substances may be different in cell 

 and medium. The intracellular pH is also not that of most media used in 

 enzyme study and may be easily changed by the addition of external 

 substrate or inhibitor. 



(b) The addition of succinate to cells may influence the endogenous respiration; 

 that is, the change in Og uptake upon adding succinate may not represent 

 accurately the rate of succinate oxidation. In other words, the correction 

 for endogenous respiration may be in error. Also, the addition of succinate 

 or malonate can secondarily alter the complex balance of the metabolic 

 systems. For example, oxalacetate is a very potent inhibitor of succinate 

 dehydrogenase and its concentration in the cell may be a controlling factor 

 in the operation of the cycle. Since oxalacetate can be formed from succin- 

 ate, and its formation inhibited by malonate, secondary changes in succinate 

 dehydrogenase activity may occur that are easily interpreted as due to 

 the direct effects of malonate. 



(c) The concentration of succinate in the cell may already be appreciable 

 and the addition of more may reduce the inhibition by malonate because 

 of the competitive nature of the inhibition. 



(d) The rate of succinate oxidation may be limited by the rate at which it 

 can enter into the cells or tissus; that is, the succinate oxidase is so active 

 that the succinate is oxidized as rapidly as it enters. In such a case, mal- 

 onate would be quite ineffective until the enzyme is inhibited sufficiently 

 to make it limiting. 



(e) The ratio (malonate) I (succinate) may be different in the cell than in the 

 medium. This could be due to differences in the permeabilities of the cell 

 membrane towards these substances for, despite the fact that succinate and 

 malonate have similar properties, many cases of differential permeabilities 

 to more closely related ions are known. It could also be due to the somewhat 

 different p/C^ values, since the internal concentrations of the active ions are 

 determined by the ionization constants (see Eq. 1-14-146 for buffered 

 cells). 



Whether these factors, or others, are responsible for the anomalous results 

 mentioned above is not known. It would be very useful to have data on the 



